There have been several types of RC oscillator. FIGS. 1 to 4 respectively show different types of RC oscillators.
However, all these oscillators have their disadvantages especially when the oscillation frequency is set to be high (such as at around 20 MHz). The speed of the RC oscillator in FIG. 1 is limited by the speed of its comparator. The RC oscillator in FIG. 2 will be affected by the threshold variation of Schmitt trigger and the trimming will be difficult since the RC is in the oscillation loop. The oscillator in FIG. 3 is easy to be trimmed but still affected by the uncertainty of switching threshold. The RC oscillator in FIG. 4 is also hard for trimming and the nonlinearity of the capacitor will have effect on the performance.
Besides, due to the varying working conditions such as different temperatures and different supply voltages, and due to the varied semiconductor process of the transistors, the transistors can not operate very stably under all circumstance, thus the oscillators can not achieve a stable high frequency.
Thus, there is a need to have a stable oscillator despite of the varying working conditions such as temperature and varied semiconductor process. There is also a need to have an oscillator that is easy to be trimmed.